The present invention relates to a combustor. More specifically, the present invention relates to a combustor, such as a combustor for use in gas turbines and the like, in which combustion is sequentially staged using a catalyst.
In a gas turbine, fuel is burned in compressed air, produced by a compressor, in one or more combustors. Traditionally, such combustors had a primary combustion zone in which an approximately stoichiometric mixture of fuel and air was formed and burned in a diffusion type combustion process under essentially homogeneous conditions. Additional air was introduced into the combustor downstream of the primary combustion zone. Although the overall fuel/air ratio was considerably less than stoichiometric, the fuel/air mixture was readily ignited at start-up and good flame stability was achieved over a wide range in firing temperatures due to the locally richer nature of the fuel/air mixture in the primary combustion zone.
Unfortunately, use of such approximately stoichiometric fuel/air mixtures resulted in very high local temperatures in the primary combustion zone. Such high temperatures promoted the formation of oxides of nitrogen ("NOx"), considered an atmospheric pollutant. It is known that combustion at lean fuel/air ratios (sometimes referred to as "lean pre-mix combustion") reduces NOx formation by reducing the maximum local gas temperatures. However, such lean mixtures are difficult to ignite and exhibit poor flame stability. Although ignition and stability can be improved by the use of diffusion type combustion from a centrally disposed pilot burner, the use of such a pilot will increase the NOx generation and, thereby, provide a lower limit on the NOx generation.
Accordingly, it has been proposed to employ a combustion catalyst in a heterogeneous combustion process. The creation of free radicals in the catalyzed fuel/air mixture has the effect of reducing the activation energy associated with the combustion reactions. Consequently, the combustion temperature can be more readily maintained below the level at which molecular nitrogen will be converted to NOx (sometimes referred to as "thermal NOx"). Moreover, even at temperatures favoring the formation of thermal NOx, the NOx generation rate will be decreased because the catalyzation of the fuel/air mixture will enhance the ability of the fuel to compete with nitrogen for the available oxygen.
Catalytic combustion has been proposed in various modes, including flowing all of the fuel/air mixture through a catalytic combustor preceded by a pilot burner or operating a catalytic combustor in parallel with a lean pre-mix combustor. In any case, the catalytic process requires that the fuel/air mixture be preheated, generally to a temperature of at least 400.degree. C. Unfortunately, the compressed air produced by a gas turbine compressor may be lower than 400.degree. C. Therefore, a diffusion type pre-heating burner is often required. However, as in a lean pre-mix type combustor, the pre-heating burner can create sufficient NOx to undermine the ability of the catalytic combustor to meet ultra-low emission requirements.
It is therefore desirable to provide a combustor capable of stable combustion with very lean mixtures of fuel and air, so as to reduce the formation of NOx, without the use of a NOx generating pilot.